This invention relates to a method for removing mercury from flue gas produced in a fossil fuel energy conversion plant. Mercury and mercury-containing compounds are present in varying amounts in fossil fuels. As the fuels are burned, the mercury enters the flue gas stream, and a portion of this mercury can ultimately be emitted from the stack. While the concentrations of mercury in the flue gas are usually low and of little concern, emitted mercury ultimately finds its way to surface water, where it is converted to more toxic compounds and can be concentrated in fish and other species in the food supply. As a result, even low levels of mercury pose a significant risk to public health, and regulations will increasingly require fossil fuel-burning plants to reduce or eliminate the amount of mercury the emit.
Various forms of mercury can exist within the flue gas, and the form of the mercury plays a key role in determining how much mercury is emitted. Mercury compounds in the fuel are converted to vapor-phase, elemental mercury in the boiler. Once the flue gas leaves the boiler, some of the elemental mercury can be oxidized to a form such as Hg+2, or alternatively, adsorbed onto fly ash. Many factors are involved, but as a consequence, the flue gas contains a mixture of varying levels of elemental, oxidized and particulate mercury.
Some of the mercury in the flue gas can be removed using the pollution control equipment often found in coal-fired power plants. Particulate mercury would be removed in equipment that is used to collect the fly ash, and electrostatic precipitators or fabric filters are examples of equipment that can accomplish this. Similarly, oxidized mercury is very soluble and is easily removed in equipment that is used to control sulfur dioxide (SO2) emissions. Wet or dry flue gas desulfurization systems are examples of equipment that can control oxidized mercury. In addition, oxidized mercury is more easily converted to particulate mercury along the flue gas path.
While particulate and elemental mercury can be controlled using methods that are well known in the art, elemental mercury is not as easily controlled. Unlike oxidized mercury, elemental mercury is not soluble and is therefore not captured in the SO2 control step. Consequently, elemental mercury tends to pass through the emission control equipment and is emitted through the stack. Thus, a common strategy for controlling mercury emissions is to oxidize the elemental mercury in the flue gas so that it can then be efficiently removed in downstream emission control equipment. Indeed, this is the primary purpose of the invention discussed in this document.
Halogens play an important role with respect to the form of the mercury in the flue gas. Halogens, which include the elements chlorine, bromine, iodine and fluorine, occur naturally in coal. They serve the important function of promoting the oxidation of elemental mercury along the flue gas path. There are a variety of mechanisms that can accomplish this. For example, some halogen-containing compounds are oxidizing agents that can directly oxidize elemental mercury. Alternatively, halogen-containing compounds can work in combination with other materials to help catalyze mercury oxidation. One example of this is the beneficial effect of halogens on mercury oxidation within a selective catalytic reduction (SCR) system. Similarly, halogen compounds can help oxidize and then retain mercury when present as a component of a sorbent material such as activated carbon.
Those familiar with the art understand that the halogens must be present in a reactive form to promote mercury oxidation. Reactive forms include the hydrogen halide (e.g., hydrobromic acid—HBr), the atomic form of the halogen (e.g., atomic bromine—Br), or the molecular form of the halogen (e.g., molecular bromine—Br2). Consequently, the prior art focuses on introducing or producing these reactive halogen forms. There are a variety of ways to accomplish this. For example, halogen-containing compounds can be added to the boiler system and/or flue gas at a location where they are thermolabile (i.e., at a location where high temperatures cause decomposition to form reactive halogen species). Alternatively, halogen-containing compounds can decomposed to reactive halogen species at high temperature in equipment external to the flue gas duct and the reactive halogen species can then be added to the flue gas at any location. Another option is to add reactive halogen species, such as HBr or Br2, directly added to the flue gas, and yet another option is for various halogen-containing species to be used in conjunction with a sorbent material such as activated carbon and the combined material can serve to catalytically oxidize and then adsorb mercury.
One way to produce reactive halogens is to use a fuel (or fuel blend) with a higher halogen content. At the high temperatures that exist within the boiler itself, the halogens are converted to reactive forms (although the proportions of the various forms depend on which halogen is being considered). The reactive halogen species leave the boiler and then help to promote mercury oxidation via the mechanisms discussed above. Extensions of this concept include the addition of halogen-containing additives to the fuel and the injection of halogen-containing additives within the furnace. As with the naturally-occurring halogens, the halogens in the additives are decomposed at high temperatures to form the reactive halogen species.
The prior art contains many examples where the elevated temperature in the boiler is used to produce reactive halogen species. For example, U.S. Pat. No. 7,507,083 B2 issued to Comrie describes a method in which sorbent compositions containing halogens such as bromine and iodine are injected onto the fuel or into the combustion chamber where the temperature is higher than about 1,500° F. Similarly, U.S. Pat. No. 6,878,358 issued to Vosteen, et al. describes a process in which a bromine compound is fed to a multistage furnace and/or the flue gas in the plant section downstream of the furnace, the temperature during contact of the bromine compound with the flue gas being at least 500° C. and preferably at least 800° C. Finally, U.S. Patent Application No. 2011/0250111 A1 filed by Pollack, et al. describes a method of removing mercury from a flue gas using molecular halogen or halogen precursors.
While the described invention is not limited by the zone where the molecular halogen or halogen precursor is introduced into the exhaust gas stream, the temperature in the injection zone must be sufficiently high to allow dissociation and/or oxidation of the elemental halogen from the halogen precursor, meaning that the temperature at the injection zone must be greater than about 1,000° F., and in some embodiments, greater than about 1,500° F.
The above examples use high temperatures along the flue gas path for decomposition of the halogen salts. It is also possible to use high-temperature systems external to the flue gas path to accomplish the same objective. Here, the reactive halogen species would be produced in a separate device and then introduced into the flue gas stream for reaction with the mercury. Examples of this approach include U.S. Patent Application No. 2007/0051239 A1 filed by Holmes, et al., which describes a method of producing atomic halogen radicals using a high-temperature/high-energy chamber for creating dissociated halogen, to be supplied to the gas stream, with or without carbonaceous material.
Similarly, U.S. Patent Application No. 2010/0284872 A1 filed by Gale, et al. describes a two-step process that first produces an acid halide by reaction of a halogen salt with steam at temperatures from about 650 to 1,000° C. (temperatures from 700 to 800° C. being preferred). This is followed by catalyzed reaction of the acid halide to the molecular halogen, which is then injected into the flue gas stream.
Many halide salts (CaBr2 and HBr being examples) cannot be thermally decomposed at temperatures below 1,000° F. That is, these salts are not thermolabile, and this property explains why they are employed at higher temperatures for the purpose of generating reactive halogen species. There are, however, halogen-containing compounds that are thermolabile at temperatures below 1,000° F. These include the ammonia halides (e.g., NH4Cl), the so-called interhalogens, and a variety of organic, halogen-containing species. These compounds share the common characteristic of decomposing into reactive halogen species as a result of being thermolabile at the temperature at the injection location.
The most obvious means for introducing reactive halogen species is to add them directly to the flue gas. As an example, U.S. Pat. No. 8,580,214 B2, filed by Moore, et al. discusses introducing a hydrogen halide selected from HBr and HI. One drawback of such technologies is that the reactive halogen species can be highly toxic, corrosive and difficult to handle.
Some halogen-containing materials can be used at temperatures below which they are thermolabile when combined with a sorbent material such as activated carbon. The use of brominated activated carbon is well documented. Here, sorbent materials can be impregnated with brominated compounds that might not otherwise be reactive at the injection location. The brominated sorbent material serves to catalytically oxidize the mercury. Then, the sorbent can retain the mercury, where it can be removed along with the sorbent in downstream particulate removal equipment.
Brominated sorbents can be produced by combining the bromine-containing compounds and the sorbent before, during or after injection into the flue gas path. Commercially-available activated carbons are brominated prior to injection. However, this is not always the case. For example, U.S. Patent Application US 2012/0308454 A1, filed by Heuter, et al. discusses a method whereby bromine-containing compounds and carbon-containing adsorbents (activated carbon or activated coke) are added to the flue gas as a mixture, or upstream relative to the flue gas flow, are brought into contact with carbon-containing adsorbents introduced in the form of a cloud of flue gas dust into the flue gas stream.
As discussed herein, there are a variety of ways to introduce reactive halogen species into a flue gas stream. Once introduced, they promote mercury oxidation through various means, resulting in a form of mercury that can be removed from the flue gas stream using equipment designed for the removal of other pollutants. Unfortunately, many of the techniques of the prior art have undesirable consequences on the operation of the equipment or the equipment itself. For example, the addition of halogens to the fuel or within the boiler can result in corrosion of metal surfaces within the boiler. Similarly, other compounds (such as ammonia) may be produced, causing equipment fouling or contributing to additional emissions from the facility. Therefore, there is a need for a method that employs reasonably non-toxic additives, injected downstream of the boiler itself, which does not require the use of supplemental adsorbents.